android/content/integrity/IntegrityUtils.java

/*
 * Copyright (C) 2019 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.content.integrity;

import static com.android.internal.util.Preconditions.checkArgument;

/**
 * Utils class for simple operations used in integrity module.
 *
 * @hide
 */
public class IntegrityUtils {

    private static final char[] HEX_CHARS = "0123456789ABCDEF".toCharArray();

    /**
     * Obtain the raw bytes from hex encoded string.
     *
     * @throws IllegalArgumentException if {@code hexDigest} is not a valid hex encoding of some
     *     bytes
     */
    public static byte[] getBytesFromHexDigest(String hexDigest) {
        checkArgument(
                hexDigest.length() % 2 == 0,
                "Invalid hex encoding " + hexDigest + ": must have even length");

        byte[] rawBytes = new byte[hexDigest.length() / 2];
        for (int i = 0; i < rawBytes.length; i++) {
            int upperNibble = hexDigest.charAt(2 * i);
            int lowerNibble = hexDigest.charAt(2 * i + 1);
            rawBytes[i] = (byte) ((hexToDec(upperNibble) << 4) | hexToDec(lowerNibble));
        }
        return rawBytes;
    }

    /** Obtain hex encoded string from raw bytes. */
    public static String getHexDigest(byte[] rawBytes) {
        char[] hexChars = new char[rawBytes.length * 2];

        for (int i = 0; i < rawBytes.length; i++) {
            int upperNibble = (rawBytes[i] >>> 4) & 0xF;
            int lowerNibble = rawBytes[i] & 0xF;
            hexChars[i * 2] = decToHex(upperNibble);
            hexChars[i * 2 + 1] = decToHex(lowerNibble);
        }
        return new String(hexChars);
    }

    private static int hexToDec(int hexChar) {
        if (hexChar >= '0' && hexChar <= '9') {
            return hexChar - '0';
        }
        if (hexChar >= 'a' && hexChar <= 'f') {
            return hexChar - 'a' + 10;
        }
        if (hexChar >= 'A' && hexChar <= 'F') {
            return hexChar - 'A' + 10;
        }
        throw new IllegalArgumentException("Invalid hex char " + hexChar);
    }

    private static char decToHex(int dec) {
        if (dec >= 0 && dec < HEX_CHARS.length) {
            return HEX_CHARS[dec];
        }

        throw new IllegalArgumentException("Invalid dec value to be converted to hex digit " + dec);
    }
}